System and method for geologically-enhanced magnetic resonance imaging logs

ABSTRACT

An interpretation method and system for NMR echo-train data in thinly laminated sequences. The invention uses geological information obtained at higher vertical resolution, such as using Electric Micro Imaging, to enhance the vertical resolution of echo-train data, and thus avoids log interpretations in which the hydrocarbon potential of the formation can be misread because low resolution logs tend to provide an average description of the formation. Such averaging is especially problematic in thinly laminated sequences that consist of highly permeable and porous sand layers and less permeable silt or essentially impermeable shale layers. In a preferred embodiment, using the additional high-resolution formation information one can estimate the typical T 2 -spectra of lithological laminae, and significantly enhance the permeability estimate in the laminated sequences. The method and system are applicable to any temporal data from other logging tools, such as the thermal neutron decay log and others. The system and method enable proper evaluation of the high potential of thinly laminated formations, which may otherwise be overlooked as low permeable formations.

FIELD OF THE INVENTION

This invention generally relates to borehole logging and logging-while-drilling (LWD), and in particular to a system and method for determining the properties of layered geological formations. A specific embodiment relates to a system and method for determining layer composition using a high-resolution log, and deriving associated high-resolution formation layer properties using time-dependent data, such as nuclear magnetic resonance (NMR) logs.

BACKGROUND OF THE INVENTION

In oil and gas exploration it is always desirable to understand the structure and properties of the geological formation surrounding a borehole, in order to determine if the formation contains hydrocarbon resources (oil and/or gas), to estimate the amount and producibility of hydrocarbon contained in the formation, and to evaluate the best options for completing the well in production. A significant aid in this evaluation is the use of wireline logging and/or logging-while-drilling (LWD) measurements of the formation surrounding the borehole (referred to collectively as “logs” or “log measurements”). Typically, one or more logging tools are lowered into the borehole and the tool readings or measurement logs are recorded as the tools traverse the borehole. These measurement logs are used to infer the desired formation properties.

In evaluating the hydrocarbon production potential of a subsurface formation, the formation is described in terms of a set of “petrophysical properties.” Such properties may include: (1) the lithology or the rock type, e.g., amount of sand, shale, limestone, or more detailed mineralogical description, (2) the porosity or fraction of the rock that is void or pore space, (3) the fluid saturations or fractions of the pore space occupied by oil, water and gas, and others. Wireline logging tools do not directly measure petrophysical properties, they measure “log properties”, for example, bulk density, electrical resistivity, acoustic velocity, or nuclear magnetic resonance (NMR) decay. Log properties are related to petrophysical properties via a set of mathematical or statistical relations, which are generally known in the art. In practice, frequently several different logging tools are combined and used simultaneously to obtain an integrated set of measurements. Thus, different tools may be used to obtain information about the same set of formation properties using different techniques, or different tools may be used to obtain information about different formation properties. Due to differences in physical measurement mechanisms and other factors, different logging tools have different volumes or zones of investigation, hence different measurement resolutions.

Subsurface formations are generally heterogeneous, so that porosity, saturation and lithology vary with position. A common example of heterogeneity is the presence in the formation of geological layers, or beds. Because logging tools have a nonzero volume of investigation, more than one layer may lie within the volume of investigation of a tool. In such cases, the petrophysical evaluation of one layer may be distorted by the presence of another layer falling within the larger volume of investigation of the tool.

The above phenomenon leads to a specific problem in the analysis of subsurface formations that include one or more underground layers, especially when the layers are thin compared with the vertical resolution of the measuring tool. Relatively thin layers (for example, less than about one foot) frequently come in groups of sometimes hundreds of layers, and have become subject to significant commercial interest because of their production potential. Any knowledge about the composition and properties of such layered formations that helps better estimate their production potential has thus become increasingly valuable.

As noted above, however, many of the standard wireline and LWD logs record data with a resolution along the borehole (generally this is the vertical resolution) - that is coarser than the geological layering of the formation. The effect of having low-resolution logs is that an interpretation of the data tends to be an average description of the formation that can seriously mislead the users of the interpretation. This “averaging” presents a particular problem in formations that contain a small fraction of thin, highly permeable and porous sand layers incorporated within a formation that consists predominantly of low permeability silts or essentially impermeable shales. In such formations the log properties that would reveal the sand layers are dominated, and thus masked, by the opposite log properties of the silts and shales. Accordingly, the averaging which is due to the low resolution of the measuring tool leads to underestimating of the production potential of the formation.

For example, many of the low resistivity and low contrast pay sands are known to be thinly laminated sand/silt or shale sequences. Most commercially available resistivity logging tools have coarse vertical resolution and fail to read resistivity of individual sand or shale layers. Instead, they read only the averaged horizontal resistivity that is low and dominated by the high conductivity of silt and shale layers, although individual sand layers can be highly resistive. As a result, the low resistivity of the layer sequence may be incorrectly interpreted as poor hydrocarbon potential of the formation.

Similar difficulty exists for NMR logging in thinly laminated formations. NMR logging tools have proved very useful in formation evaluation. Tools of this type include, for example, the centralized MRIL® tool made by NUMAR Corporation, a Halliburton company, and the sidewall CMR tool made by Schlumberger. The MRIL® tool is described, for example, in U.S. Pat. 4,710,713 to Taicher et al. and in various other publications including: “Spin Echo Magnetic Resonance Logging: Porosity and Free Fluid Index Determination,” by Miller, Paltiel, Gillen, Granot and Bouton, SPE 20561, 65th Annual Technical Conference of the SPE, New Orleans, La., Sept. 23-26, 1990; “Improved Log Quality With a Dual-Frequency Pulsed NMR Tool,” by Chandler, Drack, Miller and Prammer, SPE 28365, 69th Annual Technical Conference of the SPE, New Orleans, La., Sept. 25-28, 1994. Certain details of the structure and the use of the MRIL® tool, as well as the interpretation of various measurement parameters are also discussed in U.S. Pat. Nos. 4,717,876; 4,717,877; 4,717,878; 5,212,447; 5,280,243; 5,309,098; 5,412,320; 5,517,115, 5,557,200; 5,696,448 and 5,936,405. The structure and operation of the Schlumberger CMR tool is described, for example, in U.S. Pat. Nos. 4,939,648; 5,055,787 and 5,055,788 and further in “Novel NMR Apparatus for Investigating an External Sample,” by Kleinberg, Sezginer and Griffin, J. Magn. Reson. 97, 466-485, 1992. The content of the above patents is hereby expressly incorporated by reference for all purposes, and all non-patent references are incorporated by reference for background.

Generally, although the NMR tools respond only to a very limited zone in the radial direction, their vertical resolution is not sharp enough to identify individual layers. The vertical resolution of the tools gets worse when NMR echo-trains are stacked over multiple events to improve the signal-to-noise ratio (SNR). Consequently, the echo-trains obtained from NMR logging reflect the average properties of the laminated sequences, so that the properties of non-productive silt or shale layers may mask those of productive sand layers.

One way to address the issue of layering identification is by visual inspection of cores, which inspection can identify the boundaries for different layers. Further, there are several logging tools that are capable of identifying the geological layering and classifying the various layer types. These are imaging tools, such as the Borehole Televiewer (BHTV), CAST, the Electric Micro Imaging (EMI) Tool, the Formation Micro Scanning (FMS) Tool, and others. Similarly, other fine resolution tools such as a dipmeter, high resolution Pe log, and high frequency dielectric log HFDT, may also be used to provide high-resolution layering information, when laminae are thicker than a few inches. It has been determined that the EMI tool, for example, is capable of identifying the geological layering and also, with some care, of classifying the various layer types.

Having available geological information from high vertical resolution logs, it is desirable to estimate sand properties out of the averaged measurements. In the case of echo-train data from a NMR logging tool, it is desirable to determine the echo-trains specific to particular lithological types, which can then be used to estimate the lithology-specific T₂-distribution for the formation layers. Using well-known mathematical transformations one can then obtain much more accurate permeability estimates for each lithology type, and thus obtain a realistic evaluation of the producibility potential of the formation. Such high-resolution estimates are very desirable, because otherwise the producibility potential of certain laminated formations, which may appear to have low permeability, may be overlooked.

Prior Art

Several attempts have been made in the prior art to address the issue of improving the vertical resolution of certain logs. With reference to FIG. 1, for example, if the formation can be classified into a number of discrete layer types, then a discrete log “G” can be created that describes the lithologic layering. In this particular case, an electric micro imaging (EMI) log of the geological formation is assumed, illustrated in the left-most track in FIG. 1. The geological description provided by the EMI log can be processed to obtain image resistivity (second track from the left), to which appropriate thresholds can be applied as to obtain the discrete log G. In the example illustrated in FIG. 1, the discrete log G corresponds to a three-component layering model, consisting of clay, silt and sand. Assuming next that each layer is assigned a property-value [P], mathematically one can create a high-resolution log that describes the true formation property, denoted in a vector notation as G×P. Next, if there is a logging tool that responds linearly to the formation properties, i.e., if the tool response can be modeled as a convolution filter [F], one can create a theoretical log [T] for this tool, given by the expression:

T=F{circle around (X)}(G×P)

where {circle around (X)} denotes the convolution operation. Since the process defined in the above expression is linear, it can be re-arranged as follows:

T=L×P

where

L=F{circle around (X)}G

in which the result of the convolution term L can be regarded as the lithology actually “seen”, or “interrogated” by the logging tool, which is illustrated in the next-to-last track in FIG. 1.

Assuming that the properties of identically classified layers remain substantially constant over an interval of interest, it is possible to estimate property values P_(i) for the layers by matching the theoretical log as closely as possible to a measured log [M], as illustrated in the last track in FIG. 1. In practice, this means solving for the properties vector P in the equation:

M=L×P

As seen in the last track in FIG. 1, the theoretical log T, matches quite closely the actual measurement log M over most of the interval of interest for the illustrated example. It should be noted that in practical applications for this process to work with accuracy, the formation interval being characterized must exhibit significant variations in the fractions of the lithologic-types seen by the log.

In accordance with the approach outlined above, the following method has been proposed by one of the co-inventors of this application for use in identifying log properties of the individual layer-types:

1. Classify the lithologies into lithology-types using a high-resolution (EMI-type) log;

2. Create a high-resolution lithology log [G] in which each depth is assigned membership to one lithology-type;

3. Create of a “convolution filter” [F] appropriate to the vertical resolution of the log whose layer-properties one wishes to determine;

4. Create of the log-specific lithology log [L] by convolving the high-resolution lithology-log with the log-specific convolution filter [L=F{circle around (X)}G]; and

5. Estimate the layer-properties [P] over an interval of interest from a best-fit match between the measured log data [M] and the theoretical log constructed from the filtered lithologies—e.g., by solving M=L×P to determine P.

In a least-square sense, the solution to the equation M=L×P can be found by determining the values of P_(j) that minimize the error function: $\sum\limits_{n = 1}^{N}\left\lbrack {M_{n} - {\sum\limits_{j = 1}^{J}\left\{ {L_{nj} \cdot P_{j}} \right\}}} \right\rbrack^{2}$

where M_(n) is the value of the actual measurement at a specific depth in the logged interval (l,N), L_(nj) is the fraction of the j lithology-type seen at this depth, and P_(j) is the property-value of this lithology-type. Refinements to this process can be made, such that the layer properties are allowed to vary within the larger interval in order to ensure a better match between the theoretical and the measured logs.

Once the values of the properties vector P have been determined, it is trivial to create a high-resolution version of the theoretical logged property, using the expression T=L×P. At the user's discretion, a log can be created at different resolution by applying a user-defined filter to T.

An alternative method employing an iterative method that minimizes the error between measured logs and predicted logs computed by a forward model is described in U.S. Pat. No. 5,675,147 to Ekstrom et al. The content of this patent is hereby incorporated by reference for all purposes.

While prior art methods, such as discussed above, address to some extent the issues associated with generating high-resolution petrophysical maps, they have a number of drawbacks, some of which are discussed below, and more specifically fail to exploit information available from time-dependent log sequences, such as NMR logs.

SUMMARY OF THE INVENTION

The above limitations of current formation evaluation techniques are addressed by the system and method of the present invention, which uses a set of measured wireline and/or LWD logs and knowledge of the associated tool response models to estimate various properties of a geologic formation of interest. In the important case of a stack of thinly layered formations, the layer compositions and the formation properties inside each layer are estimated in a computationally efficient manner, using time-dependent logs. In one important aspect, the present invention is used to improve the vertical resolution of time-dependent logs, such a NMR logs.

In particular, according to this invention a system and method are proposed for the interpretation of NMR echo-tra data. Because of the improved vertical resolution, the method is especially suitable for the formation evaluation of thinly laminated sequences. To this end, geological information is obtained at a high vertical resolution, which is then used to enhance the vertical resolution of the echo-train data. In a preferred embodiment, an Electric Micro Imaging (EM) tool can be used to provide such information, although various different approaches can be used in alternative embodiments. In accordance with the invention, the high-resolution geological information is used to provide a model of the a time-dependent log data, such as a NMR log of the formation, and is compared with an actual measurement log in order to estimate lithology-specific data representations, such as the typical T₂-spectra of each lithological laminae. The lithology-specific data representations are then used to obtain petrophysical parameters of the formation, including enhanced permeability estimates in the laminated sequences. The method of this invention is applicable to any temporal data (i.e., time-varying data associated with a particular logging depth) from other logging tools, such as the thermal neutron decay log, and others.

In particular, in one aspect the present invention is a method for determining petrophysical properties of thinly layered geologic formations using nuclear magnetic resonance (NMR) echo train data, comprising: classifying layers in a portion of a geologic formation into two or more discrete layer types; providing information about layer compositions in said portion of the formation based on the classified layer types; generating an NMR echo train model using the provided information about layer compositions; providing an NMR echo train log corresponding to said portion of the formation; processing the NMR echo train log using the generated NMR echo train model to obtain lithology-specific NMR echo train data representations corresponding to said two or more discrete layer types; and determining petrophysical properties of layers in said portion of the formation from the lithology-specific NMR echo train data representations.

In specific embodiments, the method is applied to discrete layer types, which may comprise sand and shale layer types. Generally, the step of providing information about layer compositions is performed using a high-resolution tool, such as an Electric Micro Imaging (EMI) Tool. In a preferred embodiment, the actual NMR echo train at logging position z, and time t is modeled mathematically as a superposition of lithology-specific echo trains M_(i)(t); and lithological weights h_(i)(z) assigned to each lithology layer. In a preferred embodiment, the step of processing the NMR echo train log comprises computing the lithology-specific echo trains M_(i)(t) using matrix inversion.

In alternative embodiments of the invention, the NMR echo train is modeled in the T₂ distribution domain as a superposition of lithological layers characterized by their own T₂-distribution C_(i)(T₂); and lithological weights are assigned to each lithology layer. In this case, processing of the NMR echo train log comprises computing lithology-specific T₂ distribution data.

In a preferred embodiment, the petrophysical properties of layers which are determined using the method are in the group including but not limited: permeability, bulk volume irreducible (BVI) and free fluid index (FFI).

In another aspect, the invention is a geological formation interpretation system comprising a specially programmed computer having: a first memory for storing a plurality of actual measurement logs of a geological formation; a second memory for storing at least one measurement model defining a predicted measurement log based on a formation description, said formation description including two or more geological layer types and boundary layer positions; processor means for generating lithology-specific measurement log data representations corresponding to said two or more geological layer types and an actual measurement log of a geological formation; and display for communicating to a user the generated lithology-specific measurement log data representations.

In yet another aspect, the invention is a method of estimating the properties of a layered geological formation using one or more measurement logs obtained from one or more tools traversing a borehole having a number of depth intervals, comprising the steps of: building a formation description, the description comprising discrete layer types and layer compositions; generating a measurement data model using the formation description; using the generated measurement data model, processing an actual measurement log to obtain lithology-specific data representations corresponding to said discrete layer types; and determining petrophysical properties of layers in the formation from the lithology-specific data representations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of a method for creating a high-resolution log describing the properties of a formation comprising three discrete layer types, when single-valued data, such as a resistivity log is used;

FIG. 2A illustrates conceptually the multi-valued process in accordance with the present invention, using ideal, i.e., no-noise echo-decays. FIG. 2B is a table that shows the relative fractions (L_(nj)) of the three different layer types associated with each echo-decay, as illustrated in FIG. 2A;

FIGS. 3A, 3B, 3C and 3D illustrate an input lithology model and synthetic echo-train log generated in accordance with the present invention;

FIGS. 4A, 4B, 4C and 4D illustrate, for the simulated NMR log data and a three-layer type formation illustrated in FIGS. 3A, B, C and D, the average of the inverted T₂-distribution from 100 random noise realizations (FIGS. 4A, 4B and 4C) and estimated three-lithology echos with a SNR=10, where the model distributions are denoted with crossed lines.

FIGS. 5A, 5B and 5C illustrate signal (FIG. 5A), lithology (FIG. 5B) and T₂ distribution (FIG. 5C) errors for the simulated NMR signals illustrated in FIGS. 3 and 4.

FIG. 6 is a flow diagram of the method used in accordance with a preferred embodiment of the present invention;

FIG. 7 is an illustration of the NMR response in a laminated two-layer sequence consisting of sand and shale;

FIGS. 8(A-K), 9(A-C) and 10(A-K) illustrate the application of the method in accordance with the present invention in field examples.

FIG. 11 is a block diagram illustration of a system in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Prior art approaches to creating high-resolution versions of certain logged properties in layered formations, as illustrated in FIG. 1, work reasonably well for logs that have a single value at any given depth in the wellbore. However, there are several log types that record multiple values at a given depth. For example, as discussed in the background of this disclosure, nuclear-spectroscopy logs record an energy-dependent spectrum; acoustic-waveform logs record a time-dependent waveform; Nuclear Magnetic Resonance (NMR) logs record a time-dependent signal-decay.

In a preferred embodiment, this invention addresses the specific problem of how to extend high-resolution lithology property determination to multi-valued signal decay of the type provided by NMR logging tools. However, it should be understood that the process could be generalized to handle any multi-valued data gathering mechanism, such as the ones mentioned above.

Considering next for purposes of illustration NMR log data, as known in the art, it is recorded as a time-dependent signal decay, which corresponds to the relaxation properties of the formation. The raw measurement data generally cannot be interpreted directly, and must therefore be transformed into a representation that is meaningful for the analyst, i.e., in which the sought-after information is explicit. In particular, it is known in the art that the NMR time-dependent signal decay, which reflects the relaxation properties of the formation materials can be modeled as a multi-exponential process. It is now standard to extract from the raw measurement data amplitude and relaxation-time pairs that comprise the signal-decay. Thus, a theoretical signal-decay, T(t), can be described by: ${T(t)} = {\sum\limits_{k = 1}^{K}{P_{k} \cdot ^{{- t}/T_{2k}}}}$

where P_(k) is the amplitude of the component with a relaxation-time T_(2k). In practice, the T_(2k) relaxation-times are assumed, a priori, to lie in an exponential sequence—e.g., 1, 2, 4, 8, 16, 32, 64, 128, 256 . . . —and only the amplitudes associated with each “T₂-bin” are calculated. Various ways of computing and interpreting the T₂ spectrum associated with NMR relaxation data are known in the art. In a preferred embodiment, one can use the MAP algorithm disclosed in U.S. Pat. No. 5,517,115, the content of which is incorporated herein by reference for all purposes, or refer to Prammer, M. G.: “NMR Pore Size Distributions and Permeability at the Well Site,” paper SPE 28368 presented at the 1994 SPE Annual Technical Conference and Exhibition, New Orleans, Sept. 25-28, which publication is incorporated herein for background.

In accordance with this invention, since the amplitude in each T₂-bin can be assumed to be an independent variable, the T₂-bin process NMR data could be treated as if it were multiple single-valued logs obtained from materials having different relaxation properties. In a specific embodiment, one can thus approach the problem of characterizing NMR decay signals as follows:

1. Create the log-specific lithology log for the particular NMR log;

2. Derive the T₂-bin amplitudes, P_(k), for all the measured waveforms in an interval of interest;

3. Determine the characteristic T₂-bin amplitude for each lithology-type by separately treating each T₂-bin amplitude as a single-valued log.

It should be apparent that in general this approach will only work if the T₂-bin amplitudes behave linearly. In other words, if the derived T₂-bin amplitudes for a formation composed of multiple layer-types is simply the weighted-sum of the T₂-bin amplitudes for each layer-type. Generally, however, the process is only linear if the measured signal decays are essentially noise-free. Therefore, since the echo decays measured using NMR logging tools have a relatively poor signal-to-noise ratio (SNR), the T₂-bin amplitudes do not combine linearly and, more specifically, separate bins will tend to merge into a single composite bin - a process known as “bin creep”.

As a result of the non-linearity of the T₂-bin amplitudes, it is necessary that the measured data be processed at the level of the time-dependent signal-decay , where data is truly multi-valued.

In accordance with this invention, conceptually the steps in the process to handle multi-valued data are similar to those used to analyze single-value data. The difference is in the step of matching the measured and theoretical logs, since one now has to match measured and theoretical signal decays.

As stated earlier, the NMR time-dependent signal decay can be described by a multi-exponential form: ${T(t)} = {\sum\limits_{k = 1}^{K}{P_{k} \cdot ^{{- t}/T_{2k}}}}$

If a formation actually comprises several lithology-types, each with a characteristic P_(k) distribution, then the theoretical signal-decay should be simply a composite of the signal-decays in each individual layer-type, weighted according to the fraction of each lithology-type “seen” by the NMR tool. Mathematically, this is expressed as follows: ${{T(t)} = {L_{j} \cdot {\sum\limits_{k = 1}^{K}{P_{jk} \cdot ^{{- t}/T_{2k}}}}}};$ ${\sum\limits_{j = 1}^{J}L_{j}} = 1$

where P_(jk) is the T₂-bin amplitude for the k^(th) bin of the j^(th) lithology-type, L_(j) is the fraction of the j^(th) lithology “seen” by the measurement.

In practice, rather than being measured continuously, the signal-decays are recorded at discrete times t_(i), and the above expression is modified as follows: ${T\left( t_{i} \right)} = {L_{j} \cdot {\sum\limits_{k = 1}^{K}{P_{jk} \cdot ^{{- t}/T_{2k}}}}}$

Given a number of measured echo-decays over an interval of formation, the determination of the characteristic bin distributions for a number of different layers can be defined as the determination of the parameters P_(jk) that best match the measured signal-decays to the theoretical time-dependent signal-decays determined from the separate multi-exponential decays. Mathematically, the problem can be formulated as determining the elements P_(jk) that minimize the following error function: $\sum\limits_{n = 1}^{N}{\sum\limits_{i = 1}^{I}\left\lbrack {{M_{n}\left( t_{i} \right)} - {\sum\limits_{j = 1}^{J}\left\{ {L_{nj} \cdot {\sum\limits_{k = 1}^{K}{P_{jk} \cdot ^{{- t_{i}}/T_{2k}}}}} \right\}}} \right\rbrack^{2}}$

where P_(jk) is the T₂-bin amplitude for the k^(th) bin in the j^(th) lithology-type; T_(2k) is the relaxation time associated with the k^(th) bin; M_(n)(t_(i)) is the measured amplitude at time t_(i) of the n^(th) echo-decay; and L_(nj) is the fraction of the j^(th) layer causing the n^(th) echo-decay. In this example, there are K T₂-bins, J discrete layers, I echoes in each time-dependant signal-decay, and N measured signal-decays.

In accordance with this invention different methods can be used for solving for the P_(jk) elements. In principal, any one of the standard techniques described in the literature can be used. The interested reader is directed, for example, to “Numerical Recipes: The Art of Scientific Computing”, Chapter 10, Cambridge University Press, 1986, the content of which is incorporated by reference. Several commercially available numerical packages, such as MATLAB, can also be applied. It should be apparent that additional constraints on the form of P_(jk) can be applied to produce results consistent with T₂-bin estimation methods currently in use.

FIG. 2A illustrates conceptually the multi-valued process of the present invention, using ideal (no-noise) echo-decays. In the left-hand side of FIG. 2A are shown eight echo-decays, next to which are illustrated the corresponding T₂-bins. FIG. 2B is a table that shows the relative fractions (L_(nj)) of the three different layer types associated with each echo-decay. The set of three T₂-bins, and the corresponding echo-decays, illustrated in FIG. 2A show the characteristics of the three facies types—with the T₂-bins being determined by minimizing the errors between the measured and theoretical waveforms.

In accordance with the present invention, once the P_(jk) values have been determined, it is trivial for the user to generate a high-resolution log of either the theoretical waveforms or theoretical T₂-bin amplitudes. Naturally, as in the single-valued problem, the high-resolution theoretical displays can be filtered to any user-defined resolution.

ILLUSTRATIVE EMBODIMENTS

Following are several illustrative embodiments of using the method of this invention, which is referred to next as Geologically Enhanced Vertical Resolution (GEVR) method.

GEVR for Conductivity Data

The first illustration of the GEVR method of the present invention relates in fact to single-valued data, but is considered in some detail to introduce notations and to illustrate the conceptual continuity from a single-value to multi-value cases in accordance with the principles of the present invention. Consider for example an induction log data in a thinly laminated sand/shale sequence. When laminae are much thinner than the vertical resolution of an induction log, the tool registers a constant conductivity value σ^(log). It is well known that induction logs read average laminae conductivity, which can be expressed mathematically as: $\sigma^{\log} = {\sum\limits_{i}{h_{i}\sigma_{i}}}$ where ${\sum\limits_{i}h_{i}} = 1$

and σ_(i) is the lamina conductivity of the i-th lithology, whose relative volumetric abundance is h_(i). In the case of sand/shale (or any binary) sequences, only two lithological types, i.e., sand and shale, are considered. Accordingly, the above expression reduces to:

σ^(log) =h _(sand)σ_(sand) +h _(shale)σ_(shale)

h _(sand) +h _(shale)=1

For purposes of hydrocarbon exploration, of primary interest is the conductivity of the sand layers σ_(sand). The volumetric abundance of the sand h_(sand), a quantity referred to in the art as net-to-gross (N/G) ratio (which is the percentage of sand thickness in the formation) may be known either from core examination or from measurements using various logging tools. The N/G ratio is considered in further detail below. Using the above expression, if σ_(shale) is known or can be estimated, then the desired quantity σ_(sand) can be obtained from the above equation as follows: $\sigma_{sand} = \frac{\sigma^{\log} - {\left( {1 - h_{sand}} \right)\sigma_{shale}}}{h_{sand}}$

In the above expression it is assumed that σ^(log) remains essentially constant and the entire sequence is regarded as one homogeneous formation described by one constant N/G ratio.

Suppose next that σ^(log) varies significantly in such formations. If the conductivity for each lithological lamina remains relatively constant, the change of log conductivity in depth is attributed to the change in lamina abundance, i.e.: ${\sigma^{\log}(z)} = {\sum\limits_{i}{{h_{i}(z)}\sigma_{i}}}$ ${\sum\limits_{i}{h_{i}(z)}} = 1$

The lithological weight h_(i)(z) is a geological information that can be estimated from petrographical inspection of cores or from high-resolution image-logs, such as CAST and EMI. It can be also determined from high-resolution tools such as the Pe tool, HFDT dielectric log, and dip meter. Naturally, it is important to ensure that h_(i)(z) is measured with he vertical resolution of the induction log.

If the lithological weight and hence the log conductivity change sufficiently over an interval, the lamina conductivity of each lithology σ_(i) can be determined from the log by minimizing the error, $S = {\int{{z}{{{\sigma^{\log}(z)} - {\sum\limits_{i}{{h_{i}(z)}\sigma_{i}}}}}^{2}}}$

In accordance with the present invention, the lamina conductivity for the i-th layer is determined by $\sigma_{i} = {\sum\limits_{k}{\lbrack A\rbrack_{ik}^{- 1}B_{k}}}$

where [A]⁻¹ denotes matrix inversion; the elements [A]_(ik) of the matrix A are computed as follows:

[A] _(ik) =∫dzh _(i)(z)h _(k)(z)

and

B _(i) =∫dzh _(i)(z)σ^(log)(z)

If the conductivity profile at a better vertical resolution is needed, it can be constructed from the lamina conductivity simply by, ${{\overset{\sim}{\sigma}}^{\log}(z)} = {\sum\limits_{i}{{{\overset{\sim}{h}}_{i}(z)}\sigma_{i}}}$ ${\sum\limits_{i}{{\overset{\sim}{h}}_{i}(z)}} = 1$

where {tilde over (h)}_(i) (z) is the lithological weight calculated over the desired vertical resolution and can be generated from high-resolution data.

GEVR for MRIL Data

Following is an illustration of the method of this invention for multi-valued data corresponding to each depth mark. As noted above, some logging tools collect spectral data at each logging depth. For instance, Spectral Gamma Log samples gamma ray energy spectrum at each depth. Thermal Neutron Decay log measures time decay spectrum of thermal neutron population. Magnetic Resonance Imaging tool measures nuclear magnetic resonance (NMR) echo-trains, the temporal decay spectrum of polarized hydrogen nuclei. In accordance with the present invention substantially the same method of geological resolution enhancement may be applied to different multi-valued logging data.

Consider a laminated formation, where each lithological layer is characterized by its own echo train M_(i)(t). In this case, in accordance with the principles discussed above, in a preferred embodiment the echo train data M^(Log)(z;t) observed by the logging tool at a logging depth z is given by the expression: $\begin{matrix} {{M^{\log}\left( {z;t} \right)} = {\sum\limits_{i}{{h_{i}(z)}{M_{i}(t)}}}} & {{Eq}.\quad (1)} \end{matrix}$

In the above expression it is assumed that the lithology-specific echo train M_(i)(t) remains substantially constant among the layers in the formation. Variations on the order of 10-20% or even higher can be tolerated for practical purposes [CHECK]. Then, in the same way as discussed above, the lithology-specific echo train can be determined using the expression: $\begin{matrix} {{{M_{i}(t)} = {\sum\limits_{k}{\lbrack A\rbrack_{ik}^{- 1}{B_{k}(t)}}}}{where}} & {{Eq}.\quad (2)} \\ {\lbrack A\rbrack_{ik} = {\int{{{{zh}_{i}(z)}}{h_{k}(z)}}}} & {{Eq}.\quad (3)} \\ {{B_{i}(t)} = {\int{{{{zh}_{i}(z)}}{M^{\log}\left( {z;t} \right)}}}} & {{Eq}.\quad (4)} \end{matrix}$

It should be apparent that if the echo train at a better vertical resolution is needed to match to other logging data, it can be constructed from the lithology-specific echo trains using the expression $\begin{matrix} {{{\overset{\sim}{M}}^{\log}\left( {z;t} \right)} = {\sum\limits_{i}{{{\overset{\sim}{h}}_{i}(z)}{M_{i}(t)}}}} & {{Eq}.\quad (5)} \end{matrix}$

where {tilde over (h)}_(i)(z) is the lithological weight calculated over the matching vertical resolution.

In an alternative embodiment of the present invention, instead of time-domain echo train data, one case use the corresponding mapped T₂-distribution data. In this alternative embodiment, each lithological layer is characterized by its own T₂-distribution, designated C_(i)(T₂). Then, the T₂-distribution data observed by the logging tool C^(log)(z;T₂) at a logging depth z is given by $\begin{matrix} {{C^{\log}\left( {z;T_{2}} \right)} = {\sum\limits_{i}{{h_{i}(z)}{C_{i}\left( T_{2} \right)}}}} & {{Eq}.\quad (6)} \end{matrix}$

Again, it is assumed that the lithology-specific T₂-distribution remains reasonably constant among the layers in the formation. Then, in the same way as discussed above, the lithology-specific T₂-distribution can be determined by, $\begin{matrix} {{{C_{i}\left( T_{2} \right)} = {\sum\limits_{k}{\lbrack A\rbrack_{ik}^{- 1}{D_{k}\left( T_{2} \right)}}}}{where}} & {{Eq}.\quad (7)} \\ {{\lbrack A\rbrack_{ik} = {\int{{{{zh}_{i}(z)}}{h_{k}(z)}}}}{and}} & {{Eq}.\quad (8)} \\ {{D_{i}\left( T_{2} \right)} = {\int{{{{zh}_{i}(z)}}{C^{\log}\left( {z;T_{2}} \right)}}}} & {{Eq}.\quad (9)} \end{matrix}$

As before, if the T₂-distribution data at a still better vertical resolution is needed to match to other logging data, it can be constructed from the lithology-specific T₂-distributions using the expression: $\begin{matrix} {{{\overset{\sim}{C}}^{\log}\left( {z;t} \right)} = {\sum\limits_{i}{{{\overset{\sim}{h}}_{i}(z)}{C_{i}(t)}}}} & {{Eq}.\quad (10)} \end{matrix}$

where {tilde over (h)}_(i)(z) is the lithological weight calculated over the matching vertical resolution.

In accordance with another aspect of this invention, instead of considering the entire T₂-distribution in Eq. (6) above, some specific but relatively small number of quantities derived from the T₂-distribution (or the original echo-train data) may be used. For instance, the following bulk volume irreducible (BVI) and free fluid index (FFI) relations can be used in alternate embodiments of the present invention to estimate lithology-specific BVI and FFI: $\begin{matrix} {{{BVI}^{\log}(z)} = {\sum\limits_{i}{{h_{i}(z)}{BVI}_{i}}}} & {{Eq}.\quad (11)} \\ {{{FFI}^{\log}(z)} = {\sum\limits_{i}{{h_{i}(z)}{FFI}_{i}}}} & {{Eq}.\quad (12)} \end{matrix}$

In this case, similarly to Eq. (6), lithology-specific BVI and FFI can be estimated.

In accordance with the present invention it is preferable to use time-domain echo data, because in alternative embodiments the original echo-train data has to be mapped (or inverted) to T₂-distributions, in which process some information is lost, resulting in reduced processing accuracy.

The processing algorithm used in a preferred embodiment of the present invention is illustrated in FIG. 6. Processing starts at step 10. The GEVR method used in accordance with this invention is initiated using two types of data The first type of data can be designated broadly as the GEVR weight input data, and is obtained, in a preferred embodiment, in processing steps 20, 30 and 40. In particular, at step 20 in a specific embodiment a high-resolution log data, such as one obtained from a EMI tool is used to provide accurate layer composition. In a specific embodiment, illustrated also in FIG. 7A, at this processing step is determined a histogram (for example a conductivity histogram) in which individual layers identified by the high-resolution tool are assigned to two or more layer classes. FIG. 7A shows an example in which only two layer classes are used. In the following step 30, in a specific embodiment, class assignment is made by assigning a binary digit for each lithology at each depth. Once all lithology class assignments are made, in the following step 40 (or 45 in the parallel processing branch directed to different-resolution data), is computed the GEVR weight input for a given resolution, which in this embodiment is illustrated as a computation over a four-foot interval. Further, for the two-layer composition illustrated in the figure, the calculated weight is the net-to-gross (N/G) ratio. For example, in a particular case of two lithology classes of which one is sand, the N/G ratio is simply the percentage of sand thickness over the length of the formation.

The other input to the GEVR processing algorithm in accordance with the present invention is a time-dependent sequence. In a preferred embodiment illustrated at step 100 in FIG. 6, this GEVR input is a NMR echo train sequence (raw data). As shown in FIG. 6, at step 110 a standard mapping is applied from the time domain to the T₂ spectrum domain using well known algorithms such as MAP, which is described in detail, for example, in U.S. Pat. No. 5,517,115, the content of which is incorporated by reference for all purposes. The application of the MAP processing algorithm results, at step 120, in the original T₂ distribution of the input sequence.

As illustrated at steps 140, 150, both the NMR echo decay data and the GEVR weight data are processed using Eqs. (1)-(5) above to calculate lithology-specific echo trains M_(i)(t), which are then separately subjected to MAP processing in step 160 to obtain lithology-specific T₂ spectrum distributions in step 170. Not illustrated in FIG. 6, in the final step of the method various petrophysical properties, such as permeability, are estimated from the lithology-specific echo and/or T₂-spectrum information. For instance, in practical applications discussed below, the permeability estimate is shown to be significantly enhanced in the laminated sequences.

As illustrated in processing block 50, in a specific embodiment the GEVR input weights are combined with the GEVR lithology-specific echos to re-create, in step 60, theoretical echos (corresponding to the originally measured NMR echo data). In step 70 these echo sequences are passed through a mapping to obtain a theoretical GEVR output recreated T₂ distribution, which is then compared at step 90 with the original distribution (computed in step 120) to estimate the error between the recreated and the original distributions. In a specific embodiment, the error between the re-created and measured T₂ distributions (as indicated in the flow chart) is used as a confidence criteria of the model used. Ideally, (the averaged over four feet) recreated T₂ spectrum should match the original MIL T₂ spectrum reading. A comparison of the measured and the calculated lithologies in accordance with the method of this invention is illustrated in FIG. 7B, and shows very close match between the actual (measured) values and those computed using the method of this invention. In a specific embodiment, an error measure providing a quantitative expression of how well the model fits the measurement can also be provided.

In a specific embodiment, a least square error measure can be used with satisfactory results, although recursive algorithms can also be used in alternate embodiments. In accordance with another specific embodiment, if the computed error is above certain threshold, some of the model parameters can be changed, such as the size of the processed measurements and/or the lithology cut, to obtain better agreement between theoretical and actual measurement data The right-hand side in FIG. 6 illustrates another processing branch of the method, in which a different resolution, i.e., selected over a half-foot interval can be used.

Model Example

To examine how the above-illustrated GEVR processing algorithm works, a simulated model-formation consisting of three distinct lithology types, labeled a, b and c was considered. Assuming different values for the lithological weights, a synthetic echo-train data was generated for the model formation. Applying the abGEVR algorithm in accordance with this invention allowed to retrieve three echo-trains and three T₂-distributions from the synthetic echo-train data, assuming that the litho-weight values were known, as illustrated below.

A. Input lithology model and synthetic echo-train log

With reference to FIGS. 3A, 3B, 3C and 3D, suppose the existence of three lithology types, denoted for convenience σ_(i)=a, b, and c. Each lithology type has a different T₂-bin distribution, C_(i)(T₂), as shown in FIG. 3A, and corresponding lithology-echo, $\begin{matrix} {{M_{i}(t)} = {\sum\limits_{T_{2}}{{C_{i}\left( T_{2} \right)}^{{- t}/T_{2}}}}} & {{Eq}.\quad (13)} \end{matrix}$

for σ_(i)=a, b and c, as shown in FIG. 3B, where it is assumed that each lithology-type has the identical porosity of 25%.

FIG. 3C shows simulated lithology-weight functions for h_(i)(z)(σ_(i)=a, b and c) used for 97 logging depths. In this example, a simulated echo-train log was generated using the expression $\begin{matrix} {{M^{\log}\left( {z;t} \right)} = {{\sum\limits_{i}{{h_{i}(z)}{M_{i}(t)}}} + {ɛ\left( {z;t} \right)}}} & {{Eq}.\quad (14)} \end{matrix}$

where ε(z;t) is random noise added to the signal. FIG. 3D illustrates one such echo train.

B. GEVR processing

In a preferred embodiment, the lithology-weights are estimated using a fine-resolution logging tool, such as EMI and Pe, which is subject to errors. In the following example, the synthetic echo-train data was processed using the GEVR method of the present invention, assuming the lithology-weight is expressed as follows:

{tilde over (h)} _(i)(z)=h _(i)(z)+δ_(i)   (15)

where δ_(i) is the error in estimation of the litho-weight for the i-th lithology type. The resulting lithology-echo {tilde over (M)}_(i)(t) was then compared to the input model echo M_(i)(t) as shown in FIG. 4D, where the resulting echo is indicated in a cross (x) line. The litho-echo was also inverted to obtain the T₂-distribution {tilde over (C)}_(i)(T₂) for each litho-type, using conventional mapping program. {tilde over (C)}_(i)(T₂) depends on the random noise in the echo-data FIGS. 4A, 4B, and 4C show the average of the inverted T₂-distribution from 100 random noise realizations, where the model distributions are again denoted with crossed line.

C. Error analysis

In the simulation considered above, the total signal error is defined by $\begin{matrix} {\sigma^{2} = {\sum\limits_{z,t}\left( {{M^{\log}\left( {z;t} \right)} - {\sum\limits_{i}{{{\overset{\sim}{h}}_{i}(z)}{{\overset{\sim}{M}}_{i}(t)}}}} \right)^{2}}} & {{Eq}.\quad (16)} \end{matrix}$

and was computed and examined for different levels of signal error, characterized by its signal-to-noise ratio (SNR), and for different errors Si in litho-weight estimates. In order to see how the GEVR processing method used in accordance with the present invention can retrieve lithology information, also computed was the lithology-echo error and T₂-distribution error, defined respectively by, $\sigma_{M}^{2} = {\sum\limits_{i,t}\left( {{M_{i}(t)} - {{\overset{\sim}{M}}_{i}(t)}} \right)^{2}}$ and $\sigma_{C}^{2} = {\sum\limits_{i,T_{2}}\left( {{C_{i}\left( T_{2} \right)} - {{\overset{\sim}{C}}_{i}\left( T_{2} \right)}} \right)^{2}}$

In general, as the SNR increases, the error between the estimated and input lithology-echo signals decreases, as illustrated in FIG. 5A. However, when the larger error is induced in lithology-weight estimates, the larger SNR does not decrease the T₂-distribution error significantly, as shown in FIG. 5B. Finally, FIG. 5C illustrates a T₂ amplitude distribution error for the simulation example. Table 1 below further illustrates the noise properties for the simulated NMR echo example considered above.

TABLE 1 SNR = 5 SNR = 10 SNR = 20 SNR = 200 PCN = 0 Signal Err 0.0405 ± 0.0193 0.0112 ± 0.0052 0.0027 ± 0.0011 0.0004 ± 0.0007 Lith. Err 0.0094 ± 0.0064 0.0027 ± 0.0018 0.008 ± 0.0005 0.0001 ± 0.0001 T2 Amp. Err 0.0023 ± 0.0006 0.0014 ± 0.0004 0.0010 ± 0.003 0.0005 ± 0.0003 PCN = 10% Signal Err 0.2545 ± 0.0434 0.2148 ± 0.0335 0.2118 ± 0.0337 0.2059 ± 0.0277 Lith. Err 0.0172 ± 0.0128 0.0078 ± 0.0072 0.0066 ± 0.0054 0.0059 ± 0.0085 T2 Amp. Err 0.0024 ± 0.0005 0.0013 ± 0.0003 0.0010 ± 0.0003 0.0004 ± 0.0005 PCN = 20% Signal Err 0.8894 ± 0.1447 0.8335 ± 0.1456 0.8163 ± 0.1353 0.8205 ± 0.1561 Lith. Err 0.0722 ± 0.0410 0.0588 ± 0.0361 0.0572 ± 0.0381 0.0586 ± 0.0406 T2 Amp. Err 0.0028 ± 0.0005 0.0016 ± 0.0004 0.0012 ± 0.0004 0.0011 ± 0.0007

GEVR Processing for Field Data Applications

As discussed above, the application of the GEVR processing to actual NMR logging data, in accordance with the present invention is implemented using the following three-stage algorithm.

First, determine resolution-matched litho-weight h_(i)(z) from high resolution tools. In accordance with a preferred embodiment, core data, if available, or high resolution log data are used to determine the resolution-matched lithology-weight h_(I)(z). In a specific embodiment, for example, EMI resistivity, Pe, HFDT dielectric constant, and Dipmeter can be used for the high resolution log in this application. From these high resolution data, the resolution-matched litho-weight h_(I)(z) is computed, in a specific embodiment, by taking a running-average over the poorer resolution of MRIL data.

In accordance with an alternate embodiment of the present invention, the resolution-matched litho-weight h_(i)(z) information can be obtained using different methods. For example, in the absence of a high-resolution log, if a log measure is available that in some predictable manner reflects the average of the binary-layers, such that, if one knows both the properties of the layer-types and the law that predicts how multiple layers are averaged, one can deduce the relative fractions of the layers. One such approach is illustrated, for example, by G. Ostroffet al., “Integration of NMR and Conventional Log Data For Improved Petrophysical Evaluation of Shaly Sands”, SPWLA 40^(th) Annual Logging Symposium, May, 30-Jun. 3, 1999, the content of which is incorporated by reference for background.

In particular, in this approach use is made of the fact that across very thinly bedded, laminated sand-shale sequences, where the individual lamination thickness is less than that of vertical measurement resolution of the logging tool, porosity values for the individual laminations cannot be resolved using a standard NMR log. Consequently, the log total and effective log porosities φ_(t), φ_(e) represent the partial volume-weighted summations of the sand and shale porosity values φ_(shale/sand) integrated over the vertical measurement resolution of the logging tool, as illustrated in FIG. 7, and expressed in the following formulae:

Measured φ_(τ)=[(1−V _(shale))·φ_(tsand) ]+[V _(shale)·φ_(tshale)]

Measured φ_(e)=[(1−V _(shale))·φ_(esand]+[V) _(shale·φ) _(eshale])

Measured BVI=[(1−V _(shale))·BVI _(sand) ]+[V _(shale) ·BVI _(shale])

Measured BVM=[(1−V _(shale))·BVM _(tsand) ]+[V _(shale) ·BVM _(tshale])

In the above model equations V_(shale) is the laminar shale volume and not the bulk shale volume. FIG. 7 illustrates an NMR tool response in a thinly-bedded, laminated sandshale sequence. In this embodiment, the laminations are not identified directly, but rather one estimates V_(sh), which is a measure of the N/G on the scale of the logs if the formation is a laminated sand-shale sequence. If the laminar shale volume is quantified, then the pore volumetrics of the sand laminations can be re-constructed as follows:

φ_(tsand) =[Measured φ _(t)−(V _(shale)·φ_(tshale))]/[1−V_(shale)]

φ_(esand) =[Measured φ _(e)−(V _(shale)·φ_(eshale))]/[1−V _(shale)]

BVI _(sand) =[Measured BVI−(V _(shale) ·BVI _(tshale))]/[1−V _(shale)]

BVM _(sand) =[Measured BVM−(V _(shale) ·BVM _(shale))]/[1−V _(shale])

Next, determine lithology-specific echo train M_(i)(t). Once the resolution-matched litho-weight h_(I)(z) is computed, the echo-train data are inverted to obtain lithology-specific echo-train Mi(t) using the equations Eq. (2), (3), and (4), described above.

Finally, invert the echo-train to a corresponding T₂-distribution. Conventional mapping (or inversion) processing is applied to thus obtained lithology-specific echo-train data to obtain lithology-specific T₂distribution. Then, combined with litho-weights, the T₂-distribution is used for further log analysis. In some cases, it may be necessary to construct the echo-train data for an intermediate range of vertical resolution to match and compare to other logging tools of such resolutions. Eq. (5) is used to construct such resolution-matched echo-train from lithology-specific echo-train and resolution-matched litho-weights.

The algorithm flow chart for the field applications of the method in accordance with the present invention ( a modified version of the algorithm in FIG. 6) can be expressed as follows:

Collect EMI data Plot EMI Identify the zone(s) of interest Plot Conductivity (calibrated) Plot Histogram (frequency of conductivity within an interval) Model lithology type, Define litho-thresholds (cuts) Generate litho-weight curve L(i;z)

NMR echo data (from a MRIL® tool) M(z;t) Run GEVR-engine

Results: lithology-dependent echo sequences M(i;t) Map M(i;t) to T₂(i) Generate GEVR echos Map

Field Example 1: Fort Worth test well

1. GEVR processing with MRIL and EMI data

MRIL, EMI, and other conventional suite of logs were run in a Halliburton test well in Fortworth, Tex. FIG. 8A shows the EMI image log for the depth interval between 500 and 525 ft depth, indicating possible lamination. FIG. 8B shows the processed MRIL data (T₂-bin distribution and average T₂) and matched EMI resistivity data for the same depth interval. Note that the EMI resistivity data is sampled at every {fraction (1/10)}-in. interval. However, the EMI resitivity data in FIG. 8B is matched to the 4ft resolution of MRIL.

FIG. 8C is a cross plot between the matched EFI conductivity and the average T₂. Strong correlation indicated in the cross plot, except for the data from 500-525 ft interval, appears to justify the use of EMI resistivity as a high-resolution lithology measurement tool. FIG. 8D is a histogram of calibrated ESI conductivity collected at every {fraction (1/10)}-in. interval. The histogram indicates that there are two groups of resistivity data, separated at the conductivity value of 23,000 mmho/m as the cut-off. Therefore, it can be assumed that this is a binary formation.

Next, at each measurement depth, the {fraction (1/10)}-in. thick layer is classified as a silt-layer or a sand-layer if the EMI conductivity is more or less than the cut-off. Then, the sand-silt ratio is computed over ¼- or ½-ft interval as the ratio between the numbers of sand- and silt-layers. The litho-weight (sand fraction), h_(sand)(z), thus computed is plotted in FIG. 8E.

After the GEVR processing, the two litho-echos illustrated in FIG. 8F were obtained, that correspond to two lithology types in T₂distribution shown in FIG. 8G. In FIG. 8G, one lithology-type that corresponds to the high EMI conductivity has a dominant T₂ population at 4 ms, indicating the fast decay component in shale/silt lithology. On the other hand, another lithology-type corresponding to the low EMI conductivity has a dominant population at 60 ms, indicating slow decay component and larger pore size distribution. But it has a minor T₂-population also at about 2 ms. This is a reflection of the fact that resistivity alone cannot discriminate sand and shale/silt well. This is already seen in the cross plot in FIG. 8C, where some data points are found at lower T₂-average and lower EMI conductivity.

In order to check if these litho-echo trains of FIG. 8F are consistent with the original log echo-data, one can reconstruct the echo-trains using the litho-weight of FIG. 8E. The T₂ distributions obtained from reconstructed echo-trains are shown in FIG. 8H. Also computed were BVI and FFI from these reconstructed echo-train data and compared those from the log echo-train data. As illustrated in FIG. 8I, there is good agreement between the reconstructed and the actually measured data.

Once the litho-echo trains are obtained as in FIG. 8F, the BVI, FFI, total porosity, and the Coates permeability estimate are determined from these echo-train data for each lithological laminae. Then, using the weights obtained above, can be computed the GEVR-processed BVI, FFL, total porosity, and permeability estimates for the interval and compared with those from the log echo-train data in FIG. 8J and FIG. 8K. It should be apparent that the BVI, FFI, and total porosity should not change by the GEVR processing. On the other hand, permeability estimate is enhanced significantly after the GEVR processing.

2. GEVR processing with MRIL and Pe data

As noted above, the litho-weight information can be obtained from other logs. One such candidate is Pe-log (photoelectric log) whose 1-ft vertical resolution is still better than that of MRIL. FIG. 9A shows the cross plot of Pe and T₂-average for the same 500-525 ft depth interval. The Pe histogram is shown in FIG. 9B. The litho-weight was generated from the Pe data with the cutoff selected at Pe=2.2. Two lithology-types in T₂ distribution obtained by using the Pe- based litho-weight are shown in FIG. 9C.

Field Example 2: a Gulf of Mexico reservoir

The GEVR processing in accordance with the present invention was also applied to the MRIL and EMI data logged in a Gulf of Mexico reservoir, as illustrated below.

1. Depth interval X422-X427 ft

The MRIL T₂-average and EMI resistivity averaged over 4-ft are plotted in FIG. 10A. Their cross plot (FIG. 10B) indicates that EMI resistivity may be used as a lithology indicator as the difference in T₂-average often caused by lithological change. The EMI conductivity histogram of FIG. 10C may indicate the cutoff conductivity to be at 23,000 mmho/m, which is the same as in the case of Fortworth test well.

The litho-weight is derived from the EMI conductivity, as shown in FIG. 10D, where the weight is computed over ½-ft interval in the left and over 4-ft interval in the right. The GEVR processing resulted in two litho-echo trains for sand and silt laminae, which correspond to the following two distinctly different lithology-types in T₂-distribution, as shown in FIG. 10E. The more resistive laminae has T₂ components longer than 32 ms. The ore conductive laminae has only T₂-components shorter than 32 ms.

In order to check if these litho-echo trains are consistent with the original log echo-data, in this experiment applicants reconstructed echo-trains using the litho-weight of FIG. 10D. The T₂ distributions obtained from reconstructed echo-trains are shown in FIG. 10F. Also computed were BVI and FFI from these reconstructed echo-train data and compared those from the log echo-train data. The good agreement is illustrated in FIG. 10G.

Once the litho-echo trains were obtained as above, the BVI, FFI, total porosity, and the Coates permeability estimate were determined for each lithological laminae. Then, using the weights obtained above, were computed the GEVR-processed BVI, FFI, total porosity, and permeability estimates for the interval and compared with those from the log echo-train data in FIG. 10H and FIG. 10I. As noted, the BVI, FFI, and total porosity should not change by the GEVR processing. On the other hand, permeability estimate is enhanced significantly after the GEVR processing.

Plotted in FIG. 10J are EMI image data on the left track, EMI resistivity on the center track, and the T₂-distribution mapped from reconstructed echo-train on the right track. Both results reconstructed from the 4-ft average and the 112-ft average weights are shown together.

Based on the above, it should be apparent that system and method in accordance with this invention address various practical applications, including:

estimating high-resolution formation properties for 2D data (e.g., depth-varying MRILecho-train, or PSGT Nuclear spectra) rather than simple ID data (e.g., depth-varying BVI from MRIL, or C/O-Ratio from RMT);

estimating the formation properties in situations where there are more than three layers types detected, and quantified, by a high-resolution logging device.

FIG. 11 is a block diagram of a system in accordance with a specific embodiment of the present invention, which shows individual block components for controlling data collection, processing of the collected data and displaying the measurement results. In FIG. 11 a logging tool 30 comprises an MRI probe controller and pulse echo detection electronics. Not illustrated separately in the figure, a high-resolution tool, such as an EMI tool is also lowered in a borehole in the formation 109. The output signal from the tool(s) detection electronics is processed by data processor 52 to: record high-resolution data from the EMI tool and analyze the relaxation characteristics of the sample. The output of the data processor 52 is fed to parameter estimator 54. Measurement cycle controller 55 provides an appropriate control signal to the MRI probe. The processed data from the log measurements is stored in data storage 56. Data processor 52 is connected to display 58 which is capable of providing a graphical display of one or more measurement parameters, preferably superimposed on display data from data storage 56. The components of the system of the present invention shown in FIG. 11 can be implemented in hardware or software, or any combination thereof suitable for practical purposes.

While the invention has been described with reference to the preferred embodiments, it will be appreciated by those of ordinary skill in the art that modifications can be made to the structure and form of the invention without departing from its spirit and scope which is defined in the following claims. 

What is claimed is:
 1. A method for determining petrophysical properties of layered geologic formations using nuclear magnetic resonance (NMR) echo train data, comprising: classifying layers in a portion of a geologic formation into two or more discrete layer types; providing information about layer compositions in said portion of the formation based on the classified layer types; generating an NMR echo train model using the provided information about layer compositions; providing an NMR echo train log corresponding to said portion of the formation; processing the NMR echo train log using the generated NMR echo train model to obtain lithology-specific NMR echo train data representations corresponding to said two or more discrete layer types; and determining petrophysical properties of layers in said portion of the formation from the lithology-specific NMR echo train data representations.
 2. The method of claim 1 wherein said two or more discrete layer types comprise sand and shale layer types.
 3. The method of claim 1 wherein the step of providing information about layer compositions is performed using a high-resolution tool.
 4. The method of claim 3 wherein the high-resolution tool is an Electric Micro Imaging (EMI) Tool.
 5. The method of claim 1 wherein the NMR echo train model is expressed mathematically as ${M^{\log}\left( {z;t} \right)} = {\sum\limits_{i}{{h_{i}(z)}{M_{i}(t)}}}$

where M^(LOG)(z;t) is an observed echo train log at logging position z, and time t; each lithological layer is characterized by its own echo train M_(i)(t); and h_(i)(z) is a lithological weight assigned to the i-th lithology layer.
 6. The method of claim 5 wherein the step of processing the NMR echo train log comprises computing lithology specific echo trains M_(i)(t) using the expression: ${M_{i}(t)} = {\sum\limits_{k}{\lbrack A\rbrack_{ik}^{- 1}{B_{k}(t)}}}$ where[A]_(ik) = ∫zh_(i)(z)h_(k)(z) B_(i)(t) = ∫zh_(i)(z)M^(log)(z; t).


7. The method of claim 1 wherein the NMR echo train model is expressed mathematically as ${C^{\log}\left( {z;T_{2}} \right)} = {\sum\limits_{i}{{h_{i}(z)}{C_{i}\left( T_{2} \right)}}}$

where C^(Log)(z;T₂) is T₂-distribution data observed by an NMR logging tool at location z; each lithological layer is characterized by its own T₂-distribution C_(i)(T₂); and h_(i)(z) is a lithological weight assigned to the i-th lithology layer.
 8. The method of claim 7, wherein the step of processing the NMR echo train log comprises computing lithology specific T₂-distribution data using the expression: $\begin{matrix} {{{C_{i}\left( T_{2} \right)} = {\sum\limits_{k}{\left( \lbrack A\rbrack \right)_{ik}^{- 1}{D_{k}\left( T_{2} \right)}}}}{where}} \\ {{\lbrack A\rbrack_{ik} = {\int{{{{zh}_{i}(z)}}{h_{k}(z)}}}}{and}} \\ {{D_{i}\left( T_{2} \right)} = {\int{{{{zh}_{i}(z)}}{{C^{\log}\left( {z;T_{2}} \right)}.}}}} \end{matrix}$


9. The method of claim 1 wherein petrophysical properties of layers are in the group comprising: permeability, bulk volume irreducible (BVI) and free fluid index (FFI).
 10. The method of claim 1, wherein layers in the formation are thin compared with the vertical resolution of a tool used to obtain said NMR echo train data.
 11. The method of claim 1 further comprising, following the step of processing the NMR echo train log, the steps of. recreating from the lithology-specific NMR echo train data representations a composite NMR train log; comparing the composite NMR train log to the provided NMR echo train log; and computing an error measure based on the comparison.
 12. The method of claim 11 further comprising adjusting the parameters of the NMR echo train model and repeating the steps of processing the NMR echo train log, recreating a composite NMR train log, comparing the composite and the provided logs and computing an error until the computed error measure falls below a specified threshold.
 13. The method of claim 1 wherein the NMR echo train model is expressed mathematically as ${{FFI}^{\log}(z)} = {\sum\limits_{i}{{h_{i}(z)}{FFI}_{i}}}$

where FFI^(Log)(z) is the free fluid index log observed by an NMR logging tool at location z; each lithological layer is characterized by its own FFI_(i) value and h_(i)(z) is a lithological weight assigned to the i-th lithology layer.
 14. The method of claim 3 wherein the high-resolution tool is an Photoelectric (Pe) Tool.
 15. The method of claim 1 wherein the step of providing information about layer compositions is performed using a model of the petrophysical mixing relations of the layers comprising the formation.
 16. A geological formation interpretation system comprising a specially programmed computer having: a first memory for storing one or more actual time-dependent measurement logs of a geological formation; a second memory for storing at least one measurement model defining a predicted measurement log based on a formation description, said formation description including two or more geological layer types and layer compositions; processor means for generating lithology-specific measurement log data representations corresponding to said two or more geological layer types and an actual time-dependent measurement log of a geological formation; and display for communicating to a user the generated lithology-specific measurement log data representations.
 17. The system of claim 16 wherein the time-dependent measurement log is a nuclear magnetic resonance (NMR) log.
 18. The system of claim 16 wherein said formation description is provided by the output of a EMI tool.
 19. The system of claim 16 wherein said formation description is provided by the output of a Photoelectric (Pe) tool.
 20. The system of claim 16 wherein the processor means comprises matrix inversion means.
 21. The system of claim 16 wherein the processor means comprises means for mapping a NMR relaxation signal into a T₂ spectrum domain.
 22. A method of estimating the properties of a layered geological formation using at least one time-dependent measurement log obtained from a tool traversing a borehole, the method comprising the steps of: providing a formation description, the description comprising discrete layer types and layer compositions; generating a measurement data model using the formation description; using the generated measurement data model, processing an actual measurement log to obtain lithology-specific data representations corresponding to said discrete layer types; and determining petrophysical properties of layers in the formation from the lithology-specific data representations.
 23. The method of claim 22 wherein the step of providing information about layer compositions is performed using a high-resolution tool.
 24. The method of claim 23 wherein the high-resolution tool is an Electric Micro Imaging (EMI) Tool.
 25. The method of claim 23 wherein the high-resolution tool is a Photoelectric Tool.
 26. The method of claim 22 wherein the actual measurement log is a NMR log.
 27. The method of claim 22 wherein the actual measurement log is a thermal neutron decay log.
 28. The method of claim 26 wherein the NMR echo train model is expressed mathematically as ${M^{\log}\left( {z;t} \right)} = {\sum\limits_{i}{{h_{i}(z)}{M_{i}(t)}}}$

where M^(Log)(z;t) is an observed echo train log at logging position z, and time t; each lithological layer is characterized by its own echo train M_(i)(t); and h_(i)(z) is a lithological weight assigned to the i-th lithology layer.
 29. The method of claim 26, wherein layers in the formation are thin compared with the vertical resolution of a tool used to obtain said NMR echo train data.
 30. The method of claim 1 wherein the NMR echo train model is expressed mathematically as ${{BVI}^{\log}(z)} = {\sum\limits_{i}{{h_{i}(z)}{BVI}_{i}}}$

where BVI^(Log)(z) is the bulk volume irreducible log observed by an NMR logging tool at location z; each lithological layer is characterized by its own BVI_(i) value and h_(i)(z) is a lithological weight assigned to the i-th lithology layer.
 31. The system of claim 16 wherein the processor means comprises histogram computing means. 